The purpose of the proposed studies is to examine the changes in amino acid transport which occur during preimplantation development of mouse embryos. Preliminary studies suggest that transport systems A and L may disappear from two-cell embryos by the time they become blastocysts. Conversely, a previously uncharacterized system, provisionally termed B-o,+, is the predominant transport activity which blastocysts can express, but it is less conspicuous in two-cell embryos. The times during development when changes occur in the capacity of embryos to express amino acid transport activities will be determined more precisely and the biochemical mechanisms which produce the changes explored. Since the capacity to express B-o,+ activity is present in implanting blastocysts but is apparently diminished during diapause, changes in the capacity of embryos to express transport activities will also be followed as blastocysts enter and leave this apparently quiescent state. These data will be used to try to devise culture media which result in more normal development of preimplantation mouse embryos in vitro, and may therefore help improve in vitro fertilization and embryo transfer procedures. Moreover, the presence of the capacity to express B-o,+ activity in blastocysts may be a novel enough biochemical characteristic to serve as a focus on which eventually to design new birth control devices on the one hand, and tests of the suitability of individual blastocysts for embryo transfer on the other.